Pipe die method and apparatus

ABSTRACT

A die used in slip and elevator assemblies in oil and gas drilling operations for gripping tubular members while coupling and uncoupling joints, the die including electroless nickel plated, non-interrupted knurled teeth with a truncated pyramid shape and a dimple therein which reduces die surface penetration, thereby reducing wall loss due to die penetration, stress cracking, and carbon transfer leading to acid corrosion in oil and gas field tubular members and especially to corrosive resistive alloy tubular members.

1. FIELD OF THE INVENTION

[0001] The present invention relates to pipe slips and elevators in general and more particularly to the gripping dies used in such slips and elevators.

2. GENERAL BACKGROUND

[0002] Slips and elevators used primarily for lifting tubular goods, such as drill pipe or production tubing and the like, generally comprise a plurality of circumferentially spaced slip bodies called dies which are held collectively in a body which surrounds the locus of the drill pipe body and when used as slips the die body is in turn captured and held by a body known as a bowl. By means well known within the art, the device can be manipulated into position about the circumference of a length of pipe in a manner whereby the inner sides of the dies, having hardened metal gripping teeth, bite into and frictionally engage the pipe body when the weight of the pipe is applied. The slip body retains the dies in place and allows the dies to have some degree of freedom with respect to the slip or elevator body, thereby allowing conformity with the pipe body. The dies are further contoured to generally conform to the curvature of the pipe body. Such slip and elevator dies are also available with various tooth configurations that help grip the pipe. Such configurations include mud grooves that allow the pipe dies to maintain a grip even in contaminated conditions, such as when the pipe is coated with mud and oil. However, it is well known in the art that damage to the pipe occurs when the dies wear unevenly or when the die teeth become damaged, producing jagged edges, in which case stress risers may be set up in the surface of pipe which may result in premature pipe failure. The accepted method of gripping pipe in this manner depends on the ability of the die teeth to penetrate the surface of the pipe to some degree rather than apply excessive force that may crush or misshape the pipe.

[0003] The problem is compounded when such dies are used on high chromium pipe. Chromium or other nickel alloy pipe is often used in highly corrosive wells such as Hydrogen Sulfide (H₂s) gas wells. Such pipe is expensive and must be handled carefully to avoid damage to the chromium surface that attracts corrosion, thereby leading to early failure. Therefore, a new and better means of handling such chromium and nickel alloy pipe is required in order to prevent damaging the chromium pipe surfaces. A problem also exists when the hardened, high carbon steel teeth on the dies make contact with the chromium or nickel alloyed pipe, thereby transferring small amounts of carbon to the pipe at each penetration point. Such carbon transfer spots have been found to set up sites for corrosion which lead to stress cracks in the pipe. It has been found that carbon creates galvanic action, thereby hardening pipe in the same manner as hydrogen sulfide, causing brittleness of the metal. Tests on chrome pipe with salt spray have shown that any discontinuity in the surface of the pipe causes a deterioration of between 0.011-0.015 loss in pipe wall thickness per year. For example, a number 13 chrome pipe having 0.217 wall thickness with a 0.028 penetration coupled with 0.015 corrosion factor per year accelerates corrosion deterioration exponentially.

[0004] Others in the art have attempted to address the problem of handling chromium pipe and to reduce penetration, such as that disclosed by U.S. Pat. No. 5,451,084 wherein strips having hard teeth which get progressively softer along its length are held in a resilient base to allow flexibility. However, such structures fail to address the problem of sharp tooth edges resulting from mud grooves cut vertically through the tooth configuration and the problem of carbon transfer to the pipe body.

[0005] Slip, elevator and tong dies all rely on the biting action of the die's teeth into the pipe body for gripping the pipe. However, recently the industry has begun addressing these problems by attempting to reduce stress induced into the surface of the pipe through better fits, flexible die seats, etc. However, to date, such dies still generally produce penetrations of between 0.017-0.028 of an inch with 14000 ft. of pipe loading with up to 100% carbon transfer. Tests show that such high carbon deposits in the penetrations of pipe used in high corrosive wells last only a few weeks. In any case, the industry still considers die penetration of the surface of the pipe necessary. However, it is becoming essential that such penetration by the die teeth into the pipe body must be kept to an absolute minimum, generally in the order of less than {fraction (0.002/1000)} of an inch.

[0006] Unlike tong dies, slip and elevator dies are generally configured with relatively closed spaced radial teeth that are very sharp for biting into the surface of the pipe and thus maintaining purchase.

[0007] Such an arrangement is not applicable to plated pipe and tube for obvious reasons. Therefore, dies having a great many contact points spaced closer together and with special coatings are being developed in an attempt to reduce penetration on plated tubular members and still maintain purchase on the pipe when used as tong dies.

[0008] It has been found and proposed herein that pyramid shaped teeth heretofore used exclusively on tong dies are equally effective on slip and elevator dies as well. However, to maintain a sustained grip on a tubular member with sustained heavy axial loading, large pyramid shaped tong die teeth are generally sharp and usually make deep penetration within the surface of the tubular member. It is therefore claimed herein that, with modifications, a tong die having truncated pyramid shaped teeth with a dimple therein can be adapted for use in slips and elevators for sustained axial loading of pipe and tubing strings. Such slips and slip elevators include “YT” type elevators, “AU” long, “E” and “F” type slip spiders

3. SUMMARY OF THE PRESENT INVENTION

[0009] The present invention addresses the issues raised by the above discussion. Since it has been established that, generally speaking, pipe dies penetrate the surface of the pipe in order to maintain a positive grip and thus avoid crushing the pipe and it is essential that this penetration be kept to a minimum. In view of the forgoing conclusions the concept of the present invention is therefore to provide dies which have a minimum number of sharp edges, which tend to break and/or dig into the pipe or tubular body, make minimum penetration, and provide a hard, non-carbon coating over the die tooth configuration which will prevent carbon transfer to chromium or other such plated alloy pipe.

[0010] It is therefore an object of the invention to provide a pipe die having the ability to grip a tube or pipe with a minimum penetration of less than {fraction (0.002/1000)} of an inch.

[0011] Another object is to provide a gripping die that prevents carbon deposit transfer from the die to the tube or pipe.

[0012] It is still a further object of the invention to provide a pipe die with a minimum number of sharp edges which could cause cuts or otherwise mark the surface of a chromium or nickel alloy pipe body.

[0013] Yet another object is to provide an elongated, relatively thin knurled die having truncated and dimpled pyramid-coated teeth.

4. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like parts are given like reference numerals, and wherein:

[0015]FIG. 1 is an isometric view of a set of pipe slips containing the present invention;

[0016]FIG. 2 is a horizontal cross section view of FIG. 1;

[0017]FIG. 3 is a vertical cross section view of FIG. 1;

[0018]FIG. 4 is a vertical isometric view of the preferred embodiment of the die;

[0019]FIG. 5 is a vertical elevation view of the preferred embodiment of the die;

[0020]FIG. 6 is a horizontal cross section view of the preferred embodiment of the die;

[0021]FIG. 7 is a partial enlarged view taken from the area 7 seen in FIG. 6; and

[0022]FIG. 8 is a partial enlarged frontal elevation view of the die.

5. DESCRIPTION OF THE PREFERRED EMBODIMENT

[0023] As illustrated in FIG. 1 the new die insert 10 is one of several such dies arranged symmetrically and in pairs within the slip assembly 12 and or similarly arranged in pipe elevators (not shown) in the customary manner or adapted or customized for use with specialty elevators or slip assembly 12.

[0024] When the slip assembly 12 is closed, the die insert 10 contacts the outer surface 14 of the pipe or tube 16 in the manner shown in FIG. 2. It is advantageous for the die insert 10 to contact the pipe or tubing 16 at as many points as possible. Therefore, larger or smaller diameter pipe or tube may require more than the three sets of die inserts 10 shown in FIGS. 1 and 2. Further, it should be understood that since each slip or elevator manufacturer may have structural differences, the die inserts 10 might be designed to fit each particular type and model as required.

[0025] When the slip assembly 12 is placed within the tapered bowl 18 seen in FIG. 3, the weight of the pipe or tube 16 and any subsequent pipe or tube attached thereto tends to pull the wedge shaped slip assembly 12 tighter into the bowl 18 thus applying pressure to the die inserts 10. Therefore, the greater the weight of the pipe 16 and any connected string the greater the grip applied by the die inserts 10 to the outer surface of the pipe or tubing 16. When the first joint of pipe or tubing 16 and any subsequent joints suspended there from have been secured by the slip assembly 12, the next joint 20 of pipe suspended above the secured tubular assembly may be connected thereto, with the tubular assembly or string then suspended the slip assembly 12 is lifted from the bowl 18 and the pipe or tubing string is lowered into the well bore and the process is repeated.

[0026] It should be noted that the die inserts 10 are tapered in a manner whereby each die is slightly larger at the top of the slip assembly 12 than at the lower end. This insures a positive contact between the die insert 10 and the outer surface 14 of the pipe or tube, even with light loads.

[0027] The preferred embodiment as seen in FIG. 1, located within a typical slip assembly 12, engages the pipe or tube 16 in a longitudinal manner, thereby spreading the force applied by the die's teeth 22, seen more clearly in FIG. 4, over a large area of the pipe outer surface 14 in a uniform pattern as weight of the tubular string is increased to prevent crushing the tubular member. The die insert 10 is provided with pyramid shaped teeth that are much closer together than would be expected, for example, in tong dies where torque is required when gripping the pipe or tube. The die insert 10, as seen in FIG. 5, is relatively thin, rectangular, and elongated extending approximately the full length of a slip assembly 12 or elevator assembly. As previously mentioned the die inserts 10 are slightly thicker at the top “Ω” than at the bottom “Φ” to insure a positive grip and their longer length provides uniform pressure with fewer sharp edges than are usually associated with slip and elevator dies.

[0028] The height “Δ” of each of the raised geometric shapes herein referred to as teeth 22, as shown in cross-section profile in FIG. 6, are relatively small as compared to tong die teeth. This helps prevent deep tooth penetration of the pipe surface under high compressive loads and the high number of teeth per square inch, further provides even distribution of pressure on the pipe member 16 seen in FIG. 2.

[0029] The slip 12 and elevator assembly shown in FIG. 1 illustrate how the die inserts 10 are positioned, with each die having a partial radial surface in contact with the pipe surface, radii emanating along a vertical longitudinal center line of the die set. The die inserts 10 are constructed preferably out of 8620 steel with a preferred tooth embodiment as seen in FIG'S 6 and 7, formed by knurling of the die blank 24. The crossed knurled pattern illustrated in FIG. 8 by lines a-a′ and b-b′ produces a raised diamond shape, cut on the diagonal with 30 degree spiral right and left hand circular pitch knurls having 10 TPI (teeth per inch)/20T to a depth “Δ” of 0.021 of an inch in one direction and slightly shallower depth of approximately 0.020 of an inch in the opposite direction, with a root radius of 0.007 of an inch. The die insert 10 is deburred to remove all sharp edges and heat treated to a double case depth of 0.030 to 0.040 of an inch and a hardness of Rc. of 65. The difference in knurling depth in the opposite direction produces an over-cut, which tends to break or truncate the sharp tips of each tooth and thereby dimple 30 the teeth 22 as illustrated in close-up by FIG.'S 6 and 7. The knurling process is also a far less expensive machining method for forming a gripping surface than cutting specific teeth in a pipe die, thereby making this die more economical.

[0030] A special coating or plating 26 illustrated in FIG. 7 is a 0.0001 to 0.0007 or preferably 0.0002 to 0.0007 thick coating of hard chrome or electroless nickel in solution, chemically disposed by ionic transfer. This process provides a thin, very adherent, high quality, dense chromium deposit. The deposit is ideally suited to configurations such as threads and splines where conventional platings are not practical. The coating exhibits a very high degree of hardness and withstands high temperatures. This coating or plating has proven to achieve superior corrosion and wear characteristics when used in corrosive atmospheres. It has also exhibited excellent resistance against chipping, cracking or separation from the base material.

[0031] The tooth tip 28, its dimple 30, and the plating 26 illustrated in FIG. 7 reduce the penetration of the teeth 22 drastically. The dimple and its concave edges 28 of the pyramid shaped teeth 22 form the contact surface of the die insert 10 with the surface 14 of the pipe. Since these edges are also coated, no sharp edges are exposed to break and or mar the pipe surface.

[0032] Tests indicate pipe in general and chromium pipe in particular can be held successfully with the instant slip and elevator die insert 10 with virtually no visual pipe marking and only 0.0005 penetration. In fact, simulated tests have shown that up to 14000 ft. of chromium pipe can be held suspended with the instant die 10 with virtually no pipe marking and only 0.0005 penetration with 17000 ft. of pipe. Such tests have also shown a loss of contact area on the dies of less than 5% after running 17000 ft of pipe and affecting a carbon transfer of only 1% of the contact surface area at 18,500 ft. of pipe. Such tests have also shown a loss of contact area on the dies of less than 5% and affecting a carbon transfer of only 1-2% of the contact surface 14 area of the pipe 16 after running hundreds of pipe joints with applied force exerted on the pipe or tubular. Therefore, a 0.0005 of an inch penetration and a carbon transfer rate of only 1% drastically reduce the rate of corrosion and the possibility of stress cracking that generally leads to early pipe failure.

[0033] Testing has also indicated that the handling of pipe slip 12 and elevator assembly dies plays an important role in the degree of damage done to the surface of pipe. Slip 12 and elevator assemblies must engage the pipe by clamping the slip assembly 12 shown in FIG. 1 around the pipe 16 or other such tubular member in order to bring the dies' teeth into gripping contact. The die inserts 10 make contact with the pipe 16 or other such tubular members with a minimum of slip, thus reducing scarring on the pipe. The die inserts 10 having smaller teeth with dimples 30 still provide sufficient purchase on the pipe 16, thus leaving much shallower penetrations, usually less than 0.002 of an inch, and with very little carbon transfer on the pipe surface as a result of the special coating 26.

[0034] The knurled die teeth 22 are far less likely to break than the larger curved teeth generally seen in the prior art. Having a greater number of contact points protected by a hard coating insures minimum depth penetration while maintaining sufficient grip, thus reducing the number of stress points which may cause damage to the die insert 10.

[0035] A further benefit has been found by using the present die insert 10. After each pipe run, the slip dies often are replaced and the dies returned to the manufacturer for inspection and replacement or refurbishing. A great deal of time is expended in sand blasting the dies prior to inspection for stress cracks. It has been found that the sand blasting process, which often hides surface stress cracks, is not necessary when the die insert 10 is plated or coated 26 and can be easily cleaned with solvent prior to inspection, thus reducing labor and cost. Since the plating or coating 26 reduces the stress on the dies and the die suffers less damage due to a reduced number of corners, the die inserts 10 consistently last longer, thereby further reducing costs.

[0036] The present invention therefore extends the art by proving that the need for deep penetration is not necessary and that carbon transfer can be prevented when used in slip and elevators, thus increasing chromium pipe life and reducing cost associated with tong dies.

[0037] Because many varying and different embodiments may be made within the scope of the inventive concept herein taught, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirement of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in any limiting sense. 

What is claimed is:
 1. A pipe die insert of the type generally used in slip and elevator assemblies to grip, lift and suspend tubular members in oil and gas drilling operations, the die insert comprising: a) an elongated, rectangular shaped steel die member having thickness and a length approximating that of said slip and elevator assemblies, said thickness tapering along said length, said die further having a concave face relative a longitudinal axis; b) a plurality of knurled teeth arrayed over said concave face; and c) a coating means applied to all surfaces of said die, including said knurled teeth, to prevent wear and corrosion.
 2. A pipe die according to claim 1 wherein said knurled teeth are pyramid shaped having truncated tips with a dimple therein.
 3. A pipe die according to claim 1 wherein said teeth are cross-knurled and are uniform across said concave face,
 4. A pipe die according to claim 1 wherein said coating means is hard chrome plating of electroless nickel in solution, chemically disposed by ionic transfer, having a thickness of between 0.0001 and 0.0007 of an inch.
 5. A pipe die according to claim 4 wherein said hard chrome exhibits a high resistance to wear and having an equivalent hardness in excess of 60 Rockwell “C”.
 6. A pipe die insert of the type generally used in slip and elevator assemblies to grip, lift and suspend tubular members in oil and gas drilling operations, the die insert comprising: a) an elongated, rectangular shaped steel die member having thickness and a length approximating that of said slip and elevator assemblies, said thickness tapering along said length, said die further having a concave face relative a longitudinal axis; b) a plurality of cross knurled truncated pyramid shaped teeth having a dimple therein arrayed over said concave face; and c) a hard chrome electroless plating applied to said teeth having a thickness between 0.0002-0.0004 of an inch with a hardness in excess of 60 Rockwell “C”.
 7. A method of lifting and retaining threadably coupled nickel alloy drill pipe with slip and elevator assemblies without significantly marring the surface finish thereof comprising the steps of: a) replacing dies in one of said assemblies with a compatible set of die inserts, each insert comprising: i) an elongated rectangular shaped steel die member having thickness and a length approximating that of said assemblies, said thickness tapering along said length, said die further having a concave face relative a longitudinal axis; ii) a plurality of knurled teeth arrayed over said concave face; and iii) a coating means applied to all surfaces of said die, including said knurled teeth; and b) utilizing said assemblies and said replacement set of die inserts to engage said nickel alloy pipe without significantly marring said surface finish of said pipe.
 8. The method according to claim 7 including the step of repetitiously engaging said nickel alloy pipe with said dies without transferring carbon from said dies to said pipe.
 9. The method according to claim 8 includes the step of engaging said nickel alloy pipe with said dies and applying compression thereto with a carbon transfer rate of between 1-2% of the contact surface between said dies and said pipe.
 10. A method according to claim 7 further including the step of reducing cost of inspection and increasing useful longevity of slip and elevator dies comprising the step of hard chrome plating said dies with electroless nickel in solution, chemically disposed by ionic transfer, having a thickness of between 0.0001 and 0.0004 of an inch.
 11. The method according to claim 7 further includes the step of deburring, leaving said dies without any significant sharp edges.
 12. A method according to claim 7 further comprising the step of knurling said teeth upon the interior concave surface of said slip and elevator die inserts by knurling in one direction at one depth and knurling in the opposite direction at a second depth thus producing a deformed tooth formation on said surface having a truncated point and a dimple therein.
 13. The method of knurling according to claim 12 wherein said knurling process includes the use of 30 degree left and right hand spiral, circular pitch knurl to produce a pyramid shaped tooth. 